1. Field of the Invention
The present invention generally relates to a transmitter/receiver circuit for a protection circuit, for example, a transmitter/receiver circuit for a protection circuit that performs over-current protection of a secondary battery.
2. Description of the Related Art
Recent portable devices (e.g., digital cameras) have a lithium ion battery mounted thereon as a secondary battery. Because the lithium ion battery is susceptible to overcharge and over-discharge, a protection circuit for protection from overcharge and a protection circuit for protection from over-discharge are to be provided in the lithium ion battery.
In a case where plural lithium ion batteries are connected in series and used as a single battery pack, the plural lithium ion batteries are monitored by corresponding protection circuits. Because the electrical strength (withstand voltage) of electronic components used in each protection circuit becomes the voltage of a single lithium ion battery, there is no need to use an expensive electronic component (e.g., MOS transistor having high electrical strength) for the protection circuit. Accordingly, inexpensive electronic components are used in the protection circuit.
As illustrated in FIG. 4, a protection circuit according to a related art example includes serially connected secondary batteries 1, battery status detection circuits 2, level shift circuits 3, and arithmetic circuits 4. The battery status detection circuits 2 are for detecting the status of corresponding second batteries 1 and outputting signals indicating whether the status of the corresponding secondary batteries 1 are normal or abnormal. The level shift circuits 3 are for level-shifting the signals output from the battery status detection circuits 2. The arithmetic circuits 4 are for calculating the level-shifted signals output from the level shift circuits 3. Each of the level shift circuits 3 includes a constant current circuit 6 and a switch 5. The level shift circuit 3 converts the signals output from the battery status detection circuit 2 to a size of a current and outputs a signal indicative of the size of the current. The arithmetic circuit 4 performs an OR operation between the signal output from the level shift circuit 3 and the signal output from the battery status detection circuit 2 (see, for example, Japanese Laid-Open Patent Publication No. 2005-33951).
A protection circuit according to another related art example is capable of cascade connection and includes plural serially connected batteries provided inside a block, detection circuits for monitoring voltages of each battery, output terminals A for outputting signals from the detection circuits to the outside, connection terminals B for connecting to another protection circuit having the same function as the protection circuit, and an output circuit for connecting the output terminal A and the connection terminal B. The output circuit has a function of changing an electric status between the output terminal A and the connection terminal B in accordance with an output from the detection circuits. The output circuit also has a function of transmitting a signal input from the other protection circuit to the output terminal A (see, for example, Japanese Laid-Open Patent Publication No. 2005-117780).
With the protection circuit of FIG. 4, a disconnection of a signal line between the level shift circuits 3 and the arithmetic circuits 4 cannot be detected. Depending on the design of the arithmetic circuit 4, it is possible for the arithmetic circuit 4 to achieve a fail-safe configuration by outputting signals from the arithmetic circuit 4 in the same manner as the case where the battery circuit detection circuit detects an abnormal status. However, this fail-safe configuration reduces the degree of freedom for the battery status detection circuits 2 and the logical configuration of the arithmetic circuit 4.